In vehicles, a manual or automatic transmission is equipped to convert the driving force of an internal combustion engine according to a driving state of the vehicle. Such a transmission includes a gear-type transmission and a belt-type transmission, although the gear-type is usually used since it has less loss of transmitted torque. Sometimes a manual transmission is automated to operate in the same manner as the normal manual transmission.
One example of a control method for an automatic transmission is disclosed in patent JP 3011567. In the transmission and the control method disclosed therein, a target speed for a countershaft for shift control is calculated from both the number of gear teeth of a gear being selected, and a rotating speed of an output shaft. Two clutches on opposite sides of the transmission are controlled for engagement and disengagement. The shift operation is executed when the rotating speed of the countershaft is close to the target speed. Thereby, a smooth shift operation is achieved in a short time.
Another example of a control method for an automatic transmission is disclosed in Laid-Open JP No. H05-248527. In the automatic transmission and control method disclosed therein, predetermined lower oil pressure is applied to a chamber in the clutch if a condition for engagement of the clutch is satisfied. The resulting oil pressure is traveled to a position where a piston contacts with a clutch plate. However, it is set so as to not apply pressing force. From this state, the oil pressure in the oil chamber in the clutch is raised at certain timing, so that the clutch is immediately engaged. Thereby, there is a time delay in which the piston at an initial position moves to the position where the pressing force is applied to the clutch plate when the clutch is engaged.
A further example is disclosed in Laid-Open JP No. 2001-227599. The automatic transmission disclosed therein includes input and output shafts, a main clutch to transmit driving force of an engine to the input shaft, and a shift mechanism to shift a plurality of gear trains, with the shift mechanism automatically being controlled at gear shift. The transmission has a plurality of mounting shafts on which each gear defining the gear trains is mounted. An intermediate shaft is disposed at a position other than an extending portion of the mounting shaft. A sub-clutch is disposed on the intermediate shaft and variably controls torque transmitted from the output shaft to the input shaft. The sub-clutch is accommodated in a transmission case without increasing its dimension in an axial direction. In relation to the dimension of the transmission case, a space for storing, and a layout of the transmission, compatibility with the conventional transmission is increased.
In an automatic/manual transmission (hereinafter referred to as an automatic transmission) in which the conventional manual transmission (MT) is automated, the gear shift is operated in the same manner as the normal manual transmission.
More particularly, this shift operation is explained as follows: a start clutch is disconnected at a first operation; a gear is disconnected at a second operation; a shift lever is moved into a position of the gear being selected to actuate a synchronizer at a third operation; the gears are synchronized and engaged at a fourth operation; and the start clutch is engaged at a fifth operation.
At this time during the shift operation with the clutch being disconnected, shift shock does not occur. Also, poor synchronizing engagement does not occur.
Whereas, for the new automatic transmission of this invention wherein the disconnection of the driving force during gear shifting is reduced, the clutch is not required to be disengaged during gear shift, which differs from the shifting operation of the conventional automatic transmission.
For example, when shifting from a first-speed into a second-speed position: at a first operation, a clutch shares (distributes) the torque, which clutch is disposed on a drive force transmitting path between the input and output shafts and is capable of variably controlling the transmitted torque; at a second operation, the first speed gear is disengaged; at a third operation, the clutch synchronizes to the speed corresponding to the second-speed gear; at a fourth operation, the second-speed is engaged after synchronization; and at a fifth operation, the clutch is disengaged.
In these operations, if the clutch, which is disposed on a path between the input and output shafts and is capable of variably controlling the transmitted torque, does not properly share the torque, the torque is still applied to the first speed gear. In this state, the disengagement of the gear results in undesirable gear shock.
In addition, if the precise synchronization is not provided during shifting to the second-speed, the gear shock or the gear grinding noise due to poor synchronization occurs.